The invention relates to a high-temperature fuel cell, in which an electrical conductor electrically connects an interconnector to the anode of an electrolyte/electrode unit.
It is known that when water is electrolyzed the electrical current breaks down the water molecules to hydrogen (H2) and oxygen (O2) A fuel cell reverses this procedure. Electrochemical combination of hydrogen (H2) and oxygen (O2) to give water is a very effective generator of electric current. This occurs without any emission of pollutants or carbon dioxide if the fuel gas used is pure hydrogen (H2). Even with an industrial fuel gas, such as natural gas or coal gas, and with air (which may also have been enriched with oxygen (O2)) instead of pure oxygen (O2), a fuel cell produces markedly lower levels of pollutants and less carbon dioxide than other energy generators in which the energy is introduced from different sources. The fuel cell principle has been implemented industrially in various ways, and indeed with various types of electrolyte and with operating temperatures of from 80xc2x0 C. to 1,000xc2x0 C.
Depending on their operating temperature, fuel cells are divided into low, medium, and high-temperature fuel cells, and these in turn have a variety of technical configurations.
In the case of a high-temperature fuel cell stack composed of a large number of high-temperature fuel cells, there is an upper interconnector, which covers the high-temperature fuel cell stack, and under this plate there are, in order, at least one contact layer, an electrolyte/electrode unit, a further contact layer, a further interconnector, etc.
The electrolyte/electrode unit here contains two electrodesxe2x80x94an anode and a cathodexe2x80x94and a solid electrolyte configured as a membrane disposed between the anode and the cathode. Each electrolyte/electrode unit here situated between two adjacent interconnectors forms, with the contact layers situated immediately adjacent to the electrolyte/electrode unit on both sides, a high-temperature fuel cell, which also includes those sides of each of the two interconnectors which are situated on the contact layers. This type of fuel cell, and others types, are known from the reference titled xe2x80x9cFuel Cell Handbookxe2x80x9d by A. J. Appleby and F. R. Foulkes, 1989, pp. 440-454, for example.
A single high-temperature fuel cell provides an operating voltage of less than one volt. Connecting a large number of adjacent high-temperature fuel cells in series can give an operating voltage of hundreds of volts from a fuel cell system. Since the current provided by a high-temperature fuel cell is highxe2x80x94up to 1,000 amperes in the case of large high-temperature fuel cellsxe2x80x94the electrical connection between the individual cells should preferably be one that gives rise to particularly low series electrical resistance under the above-mentioned conditions.
The electrical connection between two high-temperature fuel cells is provided by an interconnector, via which the anode of one high-temperature fuel cell is connected to the cathode of the next high-temperature fuel cell. The interconnector therefore has an electrical connection to the anode of one high-temperature fuel cell and to the cathode of the next high-temperature fuel cell.
The electrical connection between the anode and the interconnector, which is configured as a plate, is provided by an electrical conductor, which may take the form of a nickel grid (see, for example, German Patent DE 196 49 457 C1). It has been found that the series electrical resistance between the anode and the interconnector, when the high-temperature fuel cell is operating, is high. This has a serious adverse effect on the electrical performance of the high-temperature fuel cell stack.
It is accordingly an object of the invention to provide a high-temperature fuel cell which overcomes the above-mentioned disadvantages of the prior art devices of this general type, which avoids any relatively high series electrical resistance, even when operating under high temperatures, and to ensure high conductivity, even over prolonged periods.
With the foregoing and other objects in view there is provided, in accordance with the invention, a high-temperature fuel cell. The fuel cell contains an electrolyte/electrode unit having an anode, an interconnector having a fuel gas side, and at least two metallic functional layers applied one above another on the fuel-gas side of the interconnector. The two metallic functional layers include a first functional layer containing nickel and a second functional layer containing copper disposed below the first functional layer. An electrical conductor connects the anode to the first functional layer.
According to the invention, the object is achieved by the high-temperature fuel cell of the type with at least two metallic functional layers that are applied one above the other on the fuel-gas side of the interconnector. One of the functional layers contains nickel and the functional layer below it contains copper.
Experiments with the high-temperature fuel cell stack and appropriate modeling experiments have shown that an increase in the electrical resistance between the electrical conductor and the interconnector formed of CrFe5Y2O31 is established, even after a short operating period at operating temperatures of between 850xc2x0 C. and 950xc2x0 C. The designation CrFe5Y2O31 represents a chromium alloy that contains 5% by weight of Fe and 1% by weight of Y2O3. The increase in the electrical resistance is caused by an oxide layer that contains chromium oxide and is formed on the surface of that side of the interconnector that faces the chamber that carries the fuel gas. It also forms where the electrical conductor, for example the nickel grid, rests on the interconnector or, for example, is joined to the interconnector by a spot weld or a soldering point. If the nickel grid has been spot-welded to the interconnector, during operation, amazingly, chromium oxide even creeps beneath these contact points, which are in the form of weld spots. Chromium has a higher electrical resistance than the unoxided metals of the interconnector.
Therefore, there is an oxide layer of poor conductivity between the electrical conductor and the interconnector, which has an unfavorable influence on the series resistance of series-connected high-temperature fuel cells. The formation of chromium oxide takes place even at oxygen partial pressures of less than 10xe2x88x9218 bar. The oxygen partial pressures are also generally present in the chamber that carries the fuel gasxe2x80x94known as the fuel-gas chamber for shortxe2x80x94while the high-temperature fuel cell is operating.
In a first step, the invention is based on the idea that suppressing the formation of the oxide layer on the anode side of the interconnector avoids any relatively high series electrical resistance and ensures high conductivity even over prolonged periods. This is reliably achieved during the operation of the high-temperature fuel cell by the fact that the interconnector is protected from oxidation by a functional layer. Naturally, a functional layer of this type should not be permeable to oxygen under operating conditions. It must not have an adverse effect on the electrical connection between conductor and interconnector. Furthermore, it should be inexpensive and easy to handle.
All these conditions are met by a thin metallic functional layer that closes off the interconnector in a gas-tight manner around the contact point. However, with a functional layer of this type the problem exists that it is oxidized during the initial heating of the high-temperature fuel cell to its operating temperature. During the initial xe2x80x9cstart-upxe2x80x9d, there is generally also sufficient air in the fuel-gas chamber of the high-temperature fuel cell to oxidize an inexpensive metallic functional layer. In this case, the oxygen also reaches the interconnector. The oxygen then forms the above-described, disruptive chromium oxide layer on the interconnector.
In a second step, the invention is based on the idea that oxidation of the interconnector can be prevented if the passage of oxygen out of a metallic functional layer into the interconnector is suppressed. This is achieved if a functional layer contains a metal which satisfies the following condition: the oxide formation of a metal of the functional layer is associated with a lower chemical potential xcexc than the oxide formation of a metal which is situated directly beneath the functional layer. If an alloy or combination of different metals is situated beneath the functional layer, all these metals must satisfy the above condition.
The term chemical potential xcexc is understood as meaning the change in the free enthalpy G of a material system as a result of component B of the system being supplied or removed:             μ      B        =                            [                      ∂            G                    ]                          [                      ∂                          η              B                                ]                    ⁢      T        ,  p  ,      n    A    ,      n    C    ,  …
In this equation, T is the thermodynamic temperature, p is the pressure, nA, nB, nC . . . are the quantities of substances A, B, C, . . . This definition is taken from Rxc3x6mpps Chemielexikon, Franckhsche Verlagsbuchhandlung, 8th edition, Stuttgart 1981.
If the oxide formation of a first metal A is associated with a lower (more negative) chemical potential xcexcA than the oxide formation of a second metal B, therefore the free energy of formation xcex94GA of the oxide of the first metal A is lower (more negative) than that of the oxide of the second metal B: xcex94GA less than xcex94GB. Accordingly, an oxygen ion in the first metal A is more firmly bonded, i.e. bonded more deeply in terms of energy, than in the second metal B. Therefore, energy is required for the oxygen ion to be transferred from the oxide of the first metal A to the second metal B.
At least two metallic functional layers which are applied one on top of the other on the fuel-gas side of the interconnector, with one functional layer containing nickel and the other functional layer below it containing copper, fulfill the conditions described above, since the combination of materials leads to the high potential threshold as described above between the layers, which greatly impedes the transfer of oxygen ions from the outer nickel layer to the copper layer below it. Furthermore, both metals are inexpensive and are easy to apply to the interconnector.
In this context, it is not absolutely necessary for the two functional layers to be applied directly on top of one another. It is generally also possible for there to be a further functional layer between the nickel-containing functional layer and the copper-containing functional layer. It is also possible for a further functional layer to be present between the interconnector and the two functional layers or above the two functional layers, without the effect of the potential threshold being significantly impaired.
The invention has the result of forming a potential threshold for oxygen ions between the functional layer and the metal below it. Consequently, oxygen in the functional layer does not pass into the metal below it, or does so only to a very limited extent. Consequently, the oxidation of the interconnector during start-up and during operation of the high-temperature fuel cell is suppressed. This prevents a relatively high series electrical resistance of the high-temperature fuel cell and ensures high conductivity even over prolonged periods.
Expediently, the copper-containing functional layer is applied to the interconnector, and the nickel-containing functional layer is applied to the copper-containing functional layer. This configuration can be produced particularly easily and inexpensively.
The nickel-containing functional layer advantageously substantially contains nickel, and the copper-containing functional layer advantageously substantially contains copper. The two metals in their pure form fulfill the conditions needed to effectively protect the interconnector from oxidation.
In an advantageous configuration of the invention, the electrical conductor is directly electrically connected to the interconnector. A direct electrical connection between the electrical conductor and the interconnector is produced by the fact that the electrical conductor is, for example, welded to the interconnector. In this case, the weld extends from the electrical conductor, through both layers, to the interconnector. In the case of an electrical conductor that is connected to the interconnector in this way, the connection is mechanically stable and involves little electrical resistance.
In an alternative configuration of the invention, the electrical conductor is electrically connected to the interconnector via at least one functional layer. An electrical connection of this type between the electrical conductor and the interconnector is achieved by the electrical conductor being joined to the upper functional layer, for example a nickel layer, by a weld, for example. An alternative possibility for producing an electrical connection of this type consists in an electrical conductor that simply rests on or is soldered to the upper functional layer. In this case, all the functional layers must be electrically conductive. This configuration of the invention is particularly simple to implement.
The thickness of the uppermost functional layer is expediently from 2 xcexcm to 10 xcexcm. A functional layer of this type is very thin yet is nevertheless suitable for constructing a potential threshold between it and the metal below and effectively preventing the oxygen ions from passing into the interconnector.
The thickness of the lower functional layer is expediently from 2 xcexcm to 10 xcexcm. A layer of this thickness is very thin yet is nevertheless suitable for constructing a potential threshold between it and the functional layer and effectively preventing the oxygen ions from passing into the interconnector.
Advantageously, at least one of the two functional layers is applied chemically, by electro-deposition, by a PVD process or by a CVD process. These processes are inexpensive and easy to carry out. These processes allow the interconnector to be coated on one side. The fuel-gas side of the interconnector should be covered over the entire surface in the region around a contact point. When coating by a physical vapor deposition (PVD) process, the material of the layer in question is applied from the vapor phase. This is achieved, for example, by sputtering, electron-beam vaporization or laser-beam vaporization. The coating temperature is less than 500xc2x0 C.
An alternative to the PVD process is a chemical vapor deposition (CVD) process. In the thermal coating process, the substance which is to form the coating is chemically generated in the vapor phase by a decomposition of starting materials and applied to the component which is to be coated.
In a further advantageous configuration of the invention, the interconnector is formed of CrFe5Y2O31, i.e. contains 94% by weight of chromium, 5% by weight of Fe and 1% by weight of Y2O3. In numerous tests, an interconnector of this type has proven suitable for operation in a high-temperature fuel cell. Furthermore, it can be coated with a metallic functional layer without problems.
In a further advantageous configuration of the invention, the electrical conductor is a nickel grid. The nickel grid may also be configured as a nickel grid assembly that contains a thinner contact grid and a thicker support grid. The electrical contact between the nickel grid (or nickel grid assembly) and the interconnector is produced by a contact point. The contact point may be in the form, for example, of a weld spot that also mechanically joins the nickel grid (or, for example, the support grid of a nickel grid assembly) to the interconnector. However, the contact point can also be produced simply by the nickel grid resting on the interconnector. Nickel is particularly appropriate, since it is not oxidized at the oxygen partial pressures of approximately 10xe2x88x9218 bar which usually prevail on the fuel-gas side while the high-temperature fuel cell is operating. Furthermore, nickel is inexpensive and easy to handle. A grid produced from nickel is elastic and ensures sufficient electrical contact between interconnector and nickel grid even when it simply rests on the interconnector. This contact is retained even in the event of temperature fluctuations within the high-temperature fuel cell.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a high-temperature fuel cell, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.